1. Field of Invention
This invention concerns a method for supercritical fluid extraction of cholesterol from untreated liquid egg yolk. In particular, this invention concerns supercritical fluid extraction of cholesterol from the untreated liquid egg yolk containing about 50% moisture using supercritical carbon dioxide solvent with or without addition of co-solvents or other agents to enhance the cholesterol extraction.
2. Related Disclosures
For their high nutrient content, healthy source of protein, good taste, colorfulness, long shelf life, ease of preparation and low cost, eggs are popular in the food industry and with consumers. However, because of the large amount of cholesterol content in egg yolk linked to cardiovascular diseases, egg consumption became restricted with the health-minded population. Since the eggs provide contain large amounts of proteins, phospholipids and other nutrients and are otherwise desirable foodstuff, there is an increased interest in providing eggs or egg products with altogether removed or greatly reduced amounts of cholesterol.
Throughout the years, many attempts were made to remove the large amounts of cholesterol from the untreated egg yolk. These attempts were largely unsuccessful because of the high water content in the egg yolk, which makes such removal impossible. Removal of cholesterol from eggs has been described, for example, in J.Amer.Oil Chem. Soc., 65:1136 (1988), EPO Patent Application EP 488181A1 (1982), and in the U.S. Pat. Nos. 4,234,619, 3,563,765 and 4,333,959. All these patents describe methods for cholesterol extraction from the dry or pre-treated egg yolk using conventional solvents, such as hydrocarbons, chlorinated hydrocarbons and alcohols.
Extraction of cholesterol and fats from dry egg yolk have been described in U.S. Pat. No. 3,563,765. The patent describes a method for production of partially defatted dry egg yolk product. In this patent, the conventional dry egg yolk solid was treated with a solvent, primarily with the non-polar solvent such as hexane, cyclohexane, heptane, tetrachloroethylene, or the like, or with the mixture of these non-polar solvents with the polar solvents such as ethyl alcohol, acetone, dioxane, and other solvents of the same type. The primary disadvantage of this method is that, in view of the fact that many organic solvents have been recently connected with tumorous growth, it is not desirable to use these solvents for food extraction. Because of the residues left in the foods after the solvent extractions, the foods submitted to the organic solvents extractions may be dangerous for human consumption. The cholesterol extraction described in the '765 patent can only be performed on dry egg yolk at temperature between 50.degree. C. to 70.degree. C. where the egg yolk proteins easily denaturated and loose their valuable properties, particularly upon reconstitution of dry egg yolk with water. Both the dry egg yolk and the use of polar solvent for extraction impair the unique characteristics of egg yolk proteins, and change the taste, appearance and texture of egg yolk.
The primary problem associated with removal of cholesterol from liquid egg yolk is the amount of water present in the untreated egg yolk. Natural, untreated egg yolk contains about 50% water and has characteristics of oil-in-water emulsion. Unless the oil-in-water emulsion characteristics are changed by chemical or possibly by mechanical means, it is impossible to extract any meaningful amount of cholesterol from the untreated liquid egg yolk.
An attempt to remove cholesterol from the liquid egg yolk where the oil-in-water emulsion is chemically disturbed is found in U.S. Pat. No. 4,333,959. That patent describes the process of reducing the amount of cholesterol in the egg yolk by reducing the pH of fresh egg yolk to destabilize egg yolk emulsion. In this patent, the pH of the egg yolk is reduced to a value around but not below 3, preferably to pH 4 to 6. The egg yolk is treated with a strong mineral acid. The destabilization is achieved by treating the egg yolk with edible vegetable oil to form a fine dispersion which, upon centrifugation, separates the egg yolk phase from the oil phase. Under this low pH conditions, the substantial amount of cholesterol is extracted into the vegetable oil with which the acidified egg yolk is treated. Similarly to the method described above, the pretreatment of the liquid egg yolk with the acid, particularly with the strong hydrochloric or phosphoric acid, poses a health risk for the public in case that the residues of these acids are not properly removed. The acid treatment also changes the properties of the egg yolk by denaturation of proteins. From a processing standpoint, adjustment of pH prior to cholesterol extraction constitutes an additional step in the process which demands a manpower, and is laborious and costly.
Two representative methods, described above, and other methods described in cited references, possess additional disadvantages. For example, used conventional solvents remove not only cholesterol but also other lipids, such as phospholipids. More importantly, they denature proteins and significantly alter functional properties (appearance, taste, texture) of the extracted egg yolk. Such changes in the properties of the egg yolk are unacceptable to the consumer. For example, phospholipids contribute many desirable properties to the egg yolk and their removal, together with the denaturation of proteins, diminishes the nutritious value of the eggs. The residues of the used conventional solvents used for their removal may cause health problems, a definite concern for consumers. The subsequent separation of cholesterol and egg phospholipids from these solvents is difficult and costly, making these methods not only unhealthy but also uneconomical. Consequently, none of the above published methods for extraction of cholesterol was ever commercially used for removal of cholesterol from the egg yolk.
Because cholesterol was found to be involved in cardiovascular diseases, there is a growing consumer demand for reduced cholesterol products. Because of their otherwise multitudinous desirable properties, eggs are the primary target for such cholesterol reduction.
It would be, therefore, extremely important to have available a method which would effectively remove substantial amounts (at least 55%) of cholesterol from various products, particularly from the eggs, without leaving behind a solvent residue or being otherwise harmful to consumer, without changing the appearance of the product, without having undesirable organoleptic properties which effect the taste of the food, or without being industrially not feasible.
The current invention offers the first such safe, fast and economical method for removal of cholesterol from untreated liquid egg yolk using a supercritical fluid extraction with carbon dioxide in the presence or absence of food grade cosolvents or other naturally occurring additives. The current invention thus provides a potential to remove more than 55% of cholesterol from the eggs using a process which is safe but also economically feasible. The process results in the egg yolk product having completely or substantially removed cholesterol without change of flavor, appearance, nutrient content, texture or other desirable properties.
In recent years, supercritical fluid extraction (SFE) has become an acceptable method for coffee and tea decaffeination, hops extraction, and flavor and spice extract production. Recent research indicates that many other additional commercial applications are likely to become utilized in food industry since the primarily used solvent for supercritical extraction, namely carbon dioxide, is safe, nontoxic, non-reactive, inexpensive, and leaves no residue in the food. The supercritical extraction technology thus provides safer and more economic alternative to produce healthier higher quality foods.
It has been previously shown that SFE can be used to effectively remove considerable amounts of cholesterol and saturated fats contained in butterfat. (Food Eng., 83 (February 1989) and Cholesterol Removal from Butterfat via Supercritical Fluid Extraction, AICHE Summer National Meeting, Denver (August 1988).
Cholesterol extraction from dried egg yolk using supercritical carbon dioxide and using carbon dioxide with cosolvents has also been described in, for example, J. Food Sci, 55:95 (1990); Extraction by Supercritical Fluids: New Products for the Food Industry; Present Status of Extraction of Supercritical Fluids of Natural Materials and Development Trends, AICHE Annual Meeting, Washington, D.C. (November 1988); and in Analysis of Cholesterol in Egg Yolk by Supercritical Fluid Chromatography, Pittsburgh Conf. New York (1990).
Additionally, the supercritical extraction of cholesterol and fats from the dry egg yolk with carbon dioxide is described in the Japanese patent JT9135847. The extraction is performed at 30.degree. to 45.degree. C. at the pressure of 130 to 150 atmospheres. However, the primary requirements for the egg yolk to be extracted with this method is that it has to be freeze-dried and has to have a moisture content no larger than 15%. The Japanese patent clearly describes the difficulty connected with extraction of cholesterol from food products having high moisture content, and concludes that it is preferred that the content of the moisture is not more than 2% to 8%.
Until now, and in agreement with Japanese Patent JP9135847, SFE of untreated liquid egg yolk has not been successfully achieved or reported. The problem connected with the cholesterol extraction from liquid yolk is the moisture present in liquid yolk. Intact untreated liquid egg yolk normally contains around 50% of water. The reduced moisture liquid egg yolk has between 1%-49% of moisture. When the moisture is above 15%, the supercritical extraction becomes very difficult if not impossible because the moisture present in the liquid egg yolk interferes with a supercritical fluid extraction technology based on finely balanced solvent phase in the critical state where the extraction of a specific material into the supercritical gas is achieved at conditions, such as temperature and pressure, specific to these material to be extracted.
To a consumer, the dried egg yolk, whether containing cholesterol or not does not replace the natural moist liquid egg yolk. While the reduced cholesterol dried egg yolk can be made liquid by reconstituting it with water, the taste, appearance and texture are forever changed. To a consumer, liquid egg yolk is always preferable to dried yolk. Regardless of how successful the reconstitution is, the reconstituted dried eggs or egg yolks always taste powdery. That feature, combined with a change in egg yolk appearance, results in diminished appeal of the dried eggs or egg yolks for the consumer. Moreover, a process for cholesterol extraction from the dried eggs or egg yolks requires lyophilization and/or spray drying, both of which processes change the egg yolk properties and raise the cost of the product. The process for cholesterol extraction from the liquid yolk, if available, would eliminate the necessity to use lyophilization.
Thus, it would be desirable to have available a method for supercritical extraction of cholesterol from liquid egg yolk which would preserve the integrity of liquid egg yolk properties but eliminate technologically difficult and costly lyophilization and spray drying connected with extraction of cholesterol from dried egg yolk or other unsafe processes connected with extraction of cholesterol from pretreated egg yolk.
It is, therefore, a primary object of this invention to provide a safe, fast and efficient method for removal of cholesterol from liquid egg yolk which method does not change the properties of the egg and is commercially feasible even for the large processing scale.